This invention relates to a device and method for wavelength locking.
There are many applications in the fields of telecommunications, medical instrumentation, optical computing and many other areas where it is required to have n optical signal having a non-varying centre wavelength within predetermined limits. In order to demultiplex a multiplexed optical signal having a plurality of optical channels, each having a unique centre wavelength, and some channel spacing between adjacent channels, it is necessary to have relatively stable channels, wherein the centre wavelengths do not vary significantly, otherwise cross-talk between adjacent channels would jeopardise the integrity of data contained therein.
Devices are commercially available that perform the function of xe2x80x9cwavelength-lockingxe2x80x9d. However, more often than not, these devices are very costly and are sensitive to temperature change. One such wavelength locking device is offered for sale by Accuwave of Santa Monica, Calif., U.S.A. By inputting a single frequency signal into the device, an output signal results with peaks on either side of a desired center wavelength. This device uses a lithium niobate block into which a volume hologram is written. The device is expensive owing to the high cost of the lithium niobate material and the difficulty in writing precision volume holograms. Furthermore, once a hologram is written into the material, the grating is fixed and cannot be changed. The Accuwave device has thermoelectric temperature control and has a variance of about 7 picometers per degree. Another limitation of this design is that it is only suitable for operating at a temperature of 15-40 degrees. U.S. Pat. No. 5,798,859 in the name of Colbourne et al., entitled Method and device for wavelength locking, describes an etalon based system. Another system in the name of the same inventor is described in U.S. Pat. No. 5,896,193 wherein an etalon is used in combination with a Bragg grating to determine within predetermined limits the wavelength of an optical signal.
A Fabry-Perot etalon has a periodic output response to broadband input light. Peaks and valleys of an etalon having a particular free spectral range correspond to predetermined wavelengths. Thus an etalon can be used as a measure of a signal""s wavelength. However, since the output response is periodic, it must be determined which period or range between two periods correspond to a particular wavelength since the output response repeats with regularity. U.S. Pat. No. 5,986,193 uses a notch response of a Bragg grating in combination with an etalon as target; this target or notch corresponding to a known wavelength can be used as a counter to a particular period corresponding to an input wavelength of light.
One requirement of a wavelength locker is that it be relatively fast, that is, that it has a substantially fast response time to an input signal. The same applies to a circuit for measuring the wavelength of an input signal. It is an object of this invention to provide a wavelength locking circuit that has a fast response time and which is highly accurate.
It is also an object of this invention to provide a wavelength locking circuit that is relatively inexpensive to manufacture.
In accordance with the invention, there is provided a device for maintaining a frequency of an optical signal, typically a narrowband optical signal, to within predetermined limits, the device comprising: a periodic filter having a periodic output response to a multi-wavelength input signal, the periodic filter optically coupled to receive a portion of the optical signal and for providing a first output response thereto;
a filter having a single-slope, or monotonically varying output response to a multi-wavelength input signal, the filter optically coupled to receive a portion of the optical signal and for providing a second output response thereto;
detector means for detecting relative intensities of the first and the second output response,
a third detector for detecting the intensity of another portion of the optical signal; and processing means for analyzing outputs from the first, second and third detector to provide an indication whether the wavelength of the optical signal is within a predetermined limit.
In an embodiment of the invention, the processing means are means for comparing the relative intensities of the first output response and the second output response with relative intensity of another portion of the input signal, and for providing an indication of the wavelength of the input signal.
The processor, or another device, may then provide an output or signal for adjusting the wavelength of the input optical signal to be locked if the wavelength is not within the predetermined limits.
For the purposes of the present specification, the definition xe2x80x9csingle-slopexe2x80x9d or xe2x80x9cmonotonicxe2x80x9d denotes, in accordance with the Webster Ninth Collegiate Dictionary, functions that either never increase or never decrease as the independent variable increases. In other words, a first derivative of such a function would be either at least zero or at most zero. Monotonic functions may be stepped i.e. may have horizontal (flat) segments. However, in order to define unequivocally the wavelength position on the periodic output function, the horizontal, or flat, segment(s) should not be longer than a single period of the periodic function. The function should be continuous over the range of periods of interest of the periodic function.
Exemplary monotonic functions for the purposes of the invention are illustrated in FIG. 4.
In a more specific embodiment of the invention, the device comprises:
a) means for splitting at least a portion of the optical signal into at least three sub-beams;
b) a first optical element having a periodic output, optically coupled to receive at least one of the at least three sub-beams;
c) a first detector optically coupled with the first optical element for detecting the intensity of light emitted from the first optical element;
d) a second optical element having a response that varies monotonically over a range of wavelengths corresponding to a plurality of periods of the first optical element, the second element being optically coupled to receive at least another of the at least three beams;
e) a second detector for detecting the intensity of light received from the second optical element;
f) a third detector for detecting the intensity of a third of the at least three sub-beams;
g) processing means for analyzing outputs from the first, second and third detector to provide an indication whether the frequency of the optical signal is within a predetermined limit, and for providing an output for adjusting the frequency of the optical signal if the frequency is not within the predetermined limits.
In an embodiment of the invention, the processing means is designed for providing an indication of wavelength that corresponds to the detected intensity of light received from the second optical element and from the third sub-beam and for utilizing the indication of wavelength and} a comparison of the relative intensity of the light detected to determine if a frequency is within the predetermined limits, and for providing an output for adjusting the frequency of the optical signal if the frequency is not within the predetermined limits.
In accordance with another aspect of the invention, there is provided a method of maintaining a frequency of an optical signal to within predetermined limits comprising the steps of:
directing a portion of the optical signal to a first optical element having a predetermined periodic output response and detecting the intensity of light emitted from the first optical element,
directing another portion of the optical signal to a second optical element having a predetermined response that varies monotonically over a range of wavelengths corresponding to a plurality of periods of the first optical element while detecting the intensity of light received from the second optical element;
detecting the intensity of another portion of the optical signal; and
analyzing the detected intensities to provide an indication whether the frequency of the optical signal is within a predetermined limit, and providing an output for adjusting the frequency of the optical signal if the frequency is not within the predetermined limits.
In an embodiment of the invention, the method comprises the steps of:
a) providing an input beam that is a portion of the optical signal;
b) splitting the input beam into at least three sub-beams;
c) directing a first of the three sub-beams to an first optical element having a predetermined periodic output response while detecting the intensity of light emitted from the first optical element;
d) directing a second of the at least three sub-beams to a second optical element having a predetermined response that varies monotonically over a range of wavelengths corresponding to a plurality of periods of the first optical element while detecting the intensity of light received from the second optical element;
e) detecting the intensity of a third of the at least three sub-beams; and
f) analyzing outputs from the first, second and third detector to provide an indication whether the frequency of the optical signal is within a predetermined limit, and providing an output for adjusting the frequency of the optical signal if the frequency is not within the predetermined limits.
In accordance with still another aspect of the invention, there is provided a method for providing an indication of a wavelength of a narrowband input signal comprising:
launching a portion of the narrowband input singnal into a periodic filter having a periodic output response to a multi-wavelength input signal and detecting intensity information related to an output signal received therefrom;
launching a portion of the narrowband input signal into a filter having a monotonically varying output response to the multi-wavelength input signal and detecting intensity information related to an output signal received therefrom; and,
processing the intensity information with intensity information related to another portion of the input signal to obtain an indication of the wavelength of the narrowband input signal.